This invention is concerned generally with magnetic bubble devices, and more particularly with a differential detection device for sensing the presence or absence of a magnetic bubble in a stream of bubbles.
A magnetic bubble detection device known in the art is an expander detector which utilizes the magnetoresistive effect in ferromagnetic materials such as permalloy to detect the presence of a magnetic bubble. Typically, an expander detector consists of a number of permalloy elements arranged to distort the cylindrical shape of a magnetic bubble incident on the expander into a more elongated configuration. The expansion of the bubble is accomplished by positioning a number of distinct columns of permalloy elements (typically chevron-shaped elements) in a triangular arrangement on a magnetic wafer which supports the magnetic bubbles. Under the influence of an applied rotating magnetic field, magnetic bubbles in a data stream are induced to propagate under the expander. The first permalloy column which a bubble encounters includes only a few chevrons, while succeeding columns are of increasing length so that as the bubble propagates under the device it is stretched out by magnetic interaction with the lengthening columns. Detection is accomplished by sensing the electrical resistance of one of the longer permalloy columns. When an elongated magnetic bubble passes under the detector, the resistance of the detection column is altered by interation of the bubble field with the fields of the permalloy elements, thereby indicating the presence of a bubble. The bubble then continues propagating across a sequence of permalloy columns of decreasing length to return the bubble to its original cylindrical shape, whereupon it may propagate back into the data stream. The overall device, then, is a triangular-shaped array of permalloy elements which distorts a bubble into an elongated shape, so that the elongated bubble passing under one column of permalloy elements can be detected, after which the bubble is reformed into its original shape and returned to the data stream.
It has been a problem with expander detectors known in the prior art to achieve a high signal-to-noise ratio in the detector output. The noise in the output appears to have various components. Of some importance is "switching noise," generated by random switching of magnetic domains in the permalloy elements of the detector. The switching noise is thus proportional to the square root of the number of permalloy chevrons in a series column. Since the signal is directly proportional to the number of chevrons in the column, it has been the practice to increase the signal-to-noise ratio in prior detectors by using a large number of chevrons in the detection column; for example, expander detectors having a central column consisting of about 300 chevrons are used in the art. The use of large detectors, however, occupies a correspondingly large area on the magnetic wafer, some of which could otherwise be utilized for data storage in the form of circulating bubble streams.
Another source of noise is magnetic flux pick-up on the electrical connections between the detector and the electronic amplifying circuits used to produce an output signal. This flux pick-up noise typically occurs at the frequency of the applied rotating magnetic field used to propagate the magnetic bubbles around the wafer.